In the manufacture of glass, glassmaking materials are provided into a glassmelting furnace and melted into molten glass which is then poured into molds to produce products such as, for example, glass bottles. The glassmaking materials for soda-lime-silicate glass include sand, soda ash, lime, and batch oxidizers such as salt cake (calcium sulfate, CaSO4) and/or niter (sodium nitrate, NaNO3, and/or potassium nitrate, KNO3) in order to control the redox state of the glass.
The glassmaking materials are melted in the furnace by heat provided by the combustion of fuel and oxidant. Water vapor resulting from the combustion reacts with alkali oxides in the molten glass to form alkali hydroxides which vaporize out from the molten glass. These alkali hydroxides, such as sodium hydroxide, NaOH, react with furnace refractory walls and cause refractory corrosion, and also react in flue passage(s) from the furnace with sulfur dioxide, SO2, and oxygen to form sodium sulfate, Na2SO4, and other sulfate and sulfite compounds which form particulates and often require expensive electrostatic precipitators or baghouses to ensure that they are not emitted to the atmosphere.
Accelerated corrosion is experienced in silica refractory bricks in the crown of glassmelting furnaces that are converted to oxy-fuel firing (i.e. combustion in which the oxidant has a higher, often much higher, oxygen content than that of air). In particular, severe loss of silica crown is observed in some glassmelting furnaces such as in glass melting for TV panels. It is generally believed that the main cause of the accelerated corrosion is the higher concentrations associated with oxy-fuel firing.
In oxy-fuel firing nitrogen contained in the combustion air is largely absent, so the volume of the combustion products is typically reduced to ⅓ to ¼ of that of the combustion products of conventional air firing. Thus the concentrations of alkali species would increase three to four times, if the same amount of volatile alkali species are generated in conventional air firing.
Accelerated corrosion shortens the furnace life and results in costly furnace repairs. In addition, corrosion increases glass defects in some glass tanks due to dripping of slag into the glass bath. Corrosion resistant refractory bricks such as alumina and alumina-zirconia-slica (AZS) bricks have been used to alleviate this corrosion. For example, AZS is often used for the side walls and flue port walls of glass furnaces, to control the corrosion problems. Silica bricks are the most widely used refractory material for the crown of furnaces because they are lighter, less heat-conductive, and substantially less expensive than alumina and AZS bricks. Also, there is concern that increased glass defects caused by zirconia “refractory stones” might occur when AZS is used for the crown. When silica is used as the material that makes up the crown of the furnace, corrosion, which causes dripping of slag into the glass bath, does not necessarily result in glass defects. This is because silica is the main composition of glass.
It would be very desirable to provide a glassmelting method wherein silica bricks can be used to line the crown of the furnace under oxy-fuel firing and wherein volatilization of alkali species is reduced to minimize corrosion of the crown and emissions of particulates.